US8461837B2 - Magnetic resonance method and apparatus with display of data acquisition progress for a subject continuously moving through the apparatus - Google Patents
Magnetic resonance method and apparatus with display of data acquisition progress for a subject continuously moving through the apparatus Download PDFInfo
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Definitions
- the present invention concerns a method, a magnetic resonance apparatus and a computer-readable storage medium to display the progress of the acquisition of measurement data from an examination region of a patient during continuous travel of the examination region through the magnetic resonance apparatus.
- Magnetic resonance (MR) is a known modality with which images of the inside of an examination subject can be generated.
- the examination subject is positioned in a comparably strong, static, homogeneous basic magnetic field (field strengths of 0.2 Tesla to 7 Tesla or more) in a magnetic resonance apparatus so that its nuclear spins orient along the basic magnetic field.
- Radio-frequency excitation pulses are radiated into the examination subject, that cause the nuclear spins to behave so as to emit magnetic resonance signals that are measured and MR images are reconstructed based thereon.
- rapidly-switched (activated) magnetic gradient fields are superimposed on the basic magnetic field.
- the acquired measurement data are digitized and stored as complex numerical values in a k-space matrix.
- An associated MR image can be reconstructed from the k-space matrix populated with such values, for example by means of a multidimensional Fourier transformation.
- the examination subject can be living (for example an animal or a patient) or inanimate (for example a sample or a phantom).
- Magnetic resonance apparatuses with a support device for example a patient bed
- a support device for example a patient bed
- a drive device for example a drive device
- the patient receptacle frequently has a quite small diameter, the patient is placed on the patient bed outside of the patient receptacle, after which the patient bed can be automatically driven into the patient receptacle by means of the drive device.
- the patient or another examination subject is briefly, continuously driven through the magnetic resonance apparatus by means of the support device during the acquisition of the measurement data from an examination region of the patient, or the examination subject.
- the measured “field of view” (FOV) can be expanded in the direction of the travel direction of the support device by controlling the movement of the support device, so examination regions that are larger in the direction of the travel direction of the support device than the measurement volume of the magnetic resonance apparatus can be examined. For example, whole-body acquisitions of patients can be generated in one measurement pass. Conversely, the measurement volume in which optimally ideal measurement conditions are generated can be limited in the direction of the travel direction of the support device without limiting the total achievable FOV.
- Applied techniques for such an acquisition of measurement data can be roughly subdivided into two-dimensional (2D) axial measurements with the travel direction of the support device perpendicular to the readout direction of the measurement data, and three-dimensional (3D) techniques in which the readout direction of the measurement data is oriented parallel to the travel direction of the support device.
- 2D two-dimensional
- 3D three-dimensional
- a current overview image (for example) is calculated using what is known as a “maximum intensity projection” (MIP) from current MR images already reconstructed from the measurement data and is displayed to the operator as a projection image.
- MIP maximum intensity projection
- the display of such a projection image can therefore normally not be implemented fast enough, in particular not in real time (i.e. simultaneously with the actual progress of the acquisition of the measurement data).
- An object of the present invention is to provide a method, a magnetic resonance apparatus and a computer-readable storage medium that enables a monitoring of the progress of a measurement with examination region moving continuously in the magnetic resonance apparatus.
- the method according to the invention for the display of a progress of an acquisition of measurement data of an examination region of a patient during a continuous travel of the examination region through a magnetic resonance apparatus includes the following steps: calculate a current projection image on the basis of current measurement data acquired from central k-space during the continuous travel of the examination region, and display the currently calculated projection image.
- the calculation of the projection images on the basis of measurement data from central k-space can ensue particularly quickly and with little effort. A particularly fast display of the projection images is therefore possible.
- the projection image is calculated particularly quickly and simply from measurement data along a central k-space line (i.e. a k-space line that runs through the center of k-space) by a one-dimensional Fourier transformation along this central k-space line.
- a progress image is constructed and displayed from successive projection images calculated in series in the course of the continuous travel of the examination region. All previously calculated projection images are therefore displayed in a composite progress image, so a better overall impression of the previously occurred measurement is achieved.
- the examination region is advantageously divided up into slices and the measurement data are acquired per slice.
- a projection image can thus be calculated for each slice of the examination region. If a progress image is constructed from such projection images, one slice in the examination region corresponds to one line in the progress image.
- Lines of the progress image can be pre-populated with a pixel value of zero until a projection image corresponding to the line is calculated and the progress image is further constructed in that the line is filled with pixel values of the projection image.
- the size of the progress image is hereby maintained overall and the portion of the progress image that has already been constructed from projection images “grows” in the course of the measurement with the progress of the measurement.
- a magnetic resonance apparatus is fashioned to acquire measurement data of an examination region of a patient during a continuous travel of the examination region through the magnetic resonance apparatus and has a support device that can travel through the magnetic resonance apparatus, a support device control unit with which movement of the support device is controlled, a computer configured to implement the method as described above, and a display device that displays images generated from the acquired measurement data.
- a computer-readable storage medium is encoded with programming code/instructions that cause the method as described above to be implemented when the storage medium is loaded into a computer that is connected with a magnetic resonance apparatus, and the programming code is executed on the computer.
- FIG. 1 schematically illustrates a magnetic resonance apparatus.
- FIG. 2 is a flowchart of an exemplary embodiment of a method to acquire measurement data from an examination region of a patient during continuous travel of the examination region through a magnetic resonance apparatus for the generation of an image data set.
- FIG. 3 is a flowchart of the method to display a progress of an acquisition of measurement data of an examination region of a patient during a continuous travel of the examination region through a magnetic resonance apparatus.
- FIG. 4 is an illustrative diagram of the SMS technique.
- FIGS. 5-7 are illustrative diagrams for various exemplary embodiments of the method to acquire measurement data of an examination region of a patient during a continuous travel of the examination region through a magnetic resonance apparatus for the generation of an image data set.
- FIGS. 8-9 show examples of weighting functions for a weighted, added use of repeatedly acquired measurement data for the generation of the image data set.
- FIG. 10 show exemplary progress images at different times during the continuous travel.
- FIG. 1 schematically shows the basic design of a magnetic resonance apparatus 1 .
- MR imaging In order to examine a body by means of MR imaging, different magnetic fields that are matched to one another as precisely as possible in terms of their temporal and spatial characteristics are radiated into the body.
- a strong magnet (typically a cryomagnet 5 with a tunnel-shaped opening) that is arranged in a measurement chamber 3 shielded against radio frequencies generates a static, strong basic magnetic field 7 that typically amounts to 0.2 Tesla to 7 Tesla or more.
- An examination subject to be examined (for example a patient; not shown here) is borne on a support device 9 (for example a patient bed) that can be moved through the magnetic resonance apparatus and is positioned in the homogeneous region of the basic magnetic field 7 for an examination. Movement of the support device 9 is controllable by a support device control unit 31 of the magnetic resonance apparatus 1 .
- the excitation of nuclear spins in the examination subject ensues by magnetic radio-frequency excitation pulses that are radiated by at least one radio-frequency antenna, represented here for example as a body coil 13 .
- the radio-frequency excitation pulses are generated by a pulse generation unit 15 that is controlled by a pulse sequence control unit 17 . After an amplification by a radio-frequency amplifier 19 , they are conducted to the at least one radio-frequency antenna.
- the radio-frequency system shown here is merely schematically indicated. Typically, more than one pulse generation unit 15 , more than one radio-frequency amplifier 19 and multiple radio-frequency antennas are used in a magnetic resonance apparatus 1 .
- the magnetic resonance apparatus 1 has gradient coils 21 with which magnetic gradient fields for (among other things) selective slice excitation and for spatial coding of the measurement signal are radiated in a measurement.
- the gradient coils 21 are controlled by a gradient coil control unit 23 that, like the pulse generation unit 15 , is connected with the pulse sequence control unit 17 .
- the signals emitted by the excited nuclear spins are received by the body coil 13 and/or local acquisition coils 25 , amplified by associated radio-frequency preamplifiers 27 and further processed and digitized by an acquisition unit 29 .
- the correct signal relaying is regulated by an upstream transmission/reception diplexer 39 .
- a computer 37 that is connected with the magnetic resonance apparatus is supplied with the measurement data. From the acquired measurement data, the computer 37 generates MR images, projection images or even additional images that can be produced from the cited MR images or projection images, for example.
- the computer 37 is connected with a memory unit 35 such that the computer 37 can store, for example, intermediate results (for example using correction steps) of the processing of the measurement data in the memory unit 35 and also retrieve them again. Images generated from the measurement data can be presented to a user via an operator console 33 or be stored in the memory unit 35 .
- the operator console 33 in particular comprises an input device 33 .
- a keyboard and/or a pointer input device such as a computer mouse
- a display device 33 for example at least one monitor—to display images created from acquired measurement data.
- the computer 37 furthermore controls the individual system components, in particular during the acquisition of the measurement data.
- the computer 37 is fashioned so that the method according to the invention can be implemented with it.
- a computer-readable storage medium 40 according to the invention is installed on the computer 37 and is encoded with programming instructions that, when executed, cause a method according to the invention to be implemented by said computer 37 .
- the shown units in particular the computer 37 , the memory unit 35 and the different control units, should not necessarily be understood as a physical unit; rather, they can be composed of multiple sub-units that are possibly separately arranged spatially.
- different methods are known for acquisition of measurement data of an examination region of a patient during a continuous travel of the examination region through a magnetic resonance apparatus.
- the cited 2D axial measurements data acquisitions
- the examination region is divided into and measured in slices that are situated perpendicular to the movement direction of the support device. In the simplest case, these slices are measured sequentially in the center of the magnetic resonance apparatus.
- the sequential measurement requires a slow speed in the movement of the support device, which leads to long total measurement times and is therefore ineffective.
- TR repetition time
- One possibility to accelerate the measurement is to combine adjacent slices into slice stacks and to measure the slices of a slice stack in an interleaved manner (as in static measurement, i.e. without movement of the support device during the measurement).
- the slice stacks themselves are measured in succession.
- the measurement position follows a fixed anatomical position within the examination subject moving continuously with the support device.
- the speed with which the support device is hereby moved is selected such that a travel path during the time of the acquisition of a slice stack is equal to twice the extent of a slice stack. This results in corresponding slices in different slices stacks (for example the respective first, second, . . . slice) being measured identically. Conversely, different slices in a common slice stack are measured differently.
- corresponding k-space lines of different slices of a slice stack are measured at different positions within the measurement volume of the magnetic resonance apparatus. Due to the (normally not ideally homogeneous) measurement conditions within the measurement volume, for example inhomogeneities of the basic magnetic field and/or nonlinearities of gradient fields, such measurements at different positions lead to different distortions of the MR images created from the measurement data. Discontinuities thereby arise in complete MR images composed of the individual MR images of the different slice stacks, in particular at the slice stack boundaries, since anatomically adjacent slices that were associated with different slice stacks take up different positions within their respective slice stack.
- SMS sliding Multislice
- the spatial frequency space (known as k-space) belonging to each slice of the real measurement volume is subdivided into S segments.
- the slices of the examination region are now divided up into the p groups according to a specific pattern. If p is equal to two, for example, those slices of the examination region with even slice index are associated with the first group, and those slices of the slice stack with odd slice index are associated with the second group.
- a division of the slices into three or generally p groups ensues in an analogous manner, meaning that each third or, respectively, p-th slice of each slice stack is respectively associated with a group.
- an active volume in the measurement volume of the magnetic resonance apparatus is selected.
- the extent of the active volume along the movement direction of the continuous movement is designed in the following as the active FOV.
- the active FOV has an extent of N slice intervals d.
- the extent of a section is precisely p slice intervals d, where the slice interval d is the distance between adjacent slices. Each segment is now associated with a section of the active FOV.
- segments that contain k-space lines near the k-space center are advantageously associated with sections of the active FOV that have a small distance (in terms of absolute value) in the direction of the travel direction of the support device from the isocenter of the magnetic resonance apparatus.
- TR is thereby the repetition time of the sequence used for the acquisition, and r is a whole number that depends on the sequence type.
- TSE turbo spin echo
- EPI echoplanar imaging
- a complete segment is normally read out after a single excitation pulse and r is thus equal to one.
- gradient echo sequences such as FLASH (Fast Low Angle Shot) or TrueFISP (True Fast Imaging with Steady state Precession)
- FLASH Flust Low Angle Shot
- TrueFISP TrueFISP
- a critical requirement to be able to implement the SMS technique is that the table feed during the acquisition time TS of a segment is precisely p slice intervals d between adjacent slices of the examination region.
- the table speed is thus:
- the SMS measurement is implemented as described in the following for illustration with regard to FIG. 4 :
- the division of the slices of the examination region into slice stacks here serves merely as an illustration.
- the first slice stack St 1 of the examination region enters directly into the active FOV of the magnetic resonance apparatus at the beginning of the measurement.
- precisely p slices 1 , 2 (here p 2) of the first slice stack St 1 enter into the first section S 1 of the active FOV of the magnetic resonance apparatus during a first time interval t 1 of duration TS.
- first time interval t 1 TS
- the k-space segment that is associated with the first section S 1 of the active FOV is measured in these p slices 1 , 2 of the first slice stack St 1 .
- these p slices 1 , 2 of the first slice stack St 1 enter into the second section S 2 of the active FOV and the k-space segment that is associated with the second section S 2 of the active FOV is acquired for the p slices 1 , 2 of the first slice stack St 1 .
- the next p slices 3 , 4 of the first slice stack St 1 (generally the p slices with slice index p+1, . . . , 2p) enter into the first section 51 of the active FOV.
- the k-space segment that is associated with the first section S 1 is acquired during the second time interval t 2 etc.
- the last p slices 7 , 8 of the first slice stack St 1 (generally the p slices with slice index N ⁇ p, . . . , N) enter into the first section S 1 of the active FOV, and the first p slices 1 , 2 of the first slice stack St 1 are located in the last section S 4 of the active FOV.
- the data of the first p slices 1 , 2 of the first slice stack St 1 are subsequently completely acquired.
- t 5 5TS—the first p slices 1 , 2 of the first slice stack St 1 have left the active FOV and the first p slices 1 , 2 of the second slice stack St 2 enter into the first section S 1 of the active FOV etc. It is noted that from the S-th time interval onwards measurement data of N segments in total are acquired per time interval TS or, respectively, that N slices are excited per repetition time TR.
- sequence techniques that are compatible with the SMS technique are T1-weighted gradient echo sequences and T2-weighted turbo spin echo sequences.
- acquisition of the measurement data for an MR image follows after multiple excitation pulses (“multi-shot techniques”), and the acquisition duration per MR image is long relative to typical time constants of human breathing (these are in the range from 3-10 seconds, for example). Therefore an acquisition of measurement data in the region of the abdomen and the lungs (thus in regions of a patient that are affected by breathing motion) cannot ensue with an SMS technique without additional measures.
- Regions affected by the breathing motion of the patient are at least part of the examination region to be examined, for example in whole-body examinations (for example in what are known as “screenings” in which, for example, persons without disease symptoms are examined from head to toe for possible undetected illnesses or their precursor stages) or in other examinations of, for example, the torso or portions of the torso of a patient to be examined.
- the acquisition of the measurement data is synchronized with the quasi-periodical breathing movement so that the acquisition respectively occurs in an identical breathing phase.
- the periodic measurement pauses that thus occur are however not compatible with acquisitions given continuous travel of the support device (and therefore of the examination region). Therefore, this type of acquisition is only possible with a stationary support device. If an examination region to be examined is larger than a measurement volume of the magnetic resonance apparatus that is used, the examination region must be organized into sub-examination regions that fit into the measurement volume and are successively driven into the measurement volume (for example by means of the support device) in order to acquire respective measurement data there given a stationary support device).
- Such a step-by-step acquisition of measurement data from examination regions of an examination region given a respectively stationary support device is also possible if the patient holds his or her breath, instead of by respiratory triggering.
- Each sub-region is hereby traversed in the measurement volume, and at the start of the acquisition the patient is asked to hold his breath until the acquisition of the measurement data for this sub-examination region has concluded.
- After acquisition of the measurement data of the sub-examination region the patient can breathe until the next examination region is moved into the measurement volume and the acquisition of measurement data of this sub-examination region begins.
- the position of the sub-examination region can be shifted depending on how strongly the patient has inhaled or, respectively, exhaled before each holding of his breath, which can lead to gaps or overlaps between the examined sub-examination region in the complete MR image of the examination region that is created from the measurement data. For example, if a lesion is located in such a gap, this can be overlooked in the examination.
- sub-regions of the examination region that should be measured while the breath is held would have to be selected before the start of the measurement.
- a sub-region may only comprise whole slice stacks. That only complete slice stacks can be selected in a sub-region given whose measurement the breath should be held severely limits the freedom in this selection.
- the measurement is less efficient since a slice stack can contain slices that are affected by the breathing motion and others that are not affected or are only slightly affected by the breathing motion.
- the speed of the feed of the support device (on which the patient rests during the examination) must be selected high enough in order to have left the region of the examination region that is affected by the breathing movement promptly before resumption of the breathing of the patient.
- the temporal placement of the breath-hold interval at the beginning of the measurement it is achieved that a typically deep breathing of the patient before the long breath-hold interval and a breath-hold command to the patient from an operator attending the measurement ensues to temporally match the breath-hold interval with the measurement before the measurement, and thus the entire acquisition of the measurement data can be implemented without interruption under continuous travel of the support device.
- FIG. 2 shows a flowchart of an exemplary embodiment of an additional method to acquire measurement data of an examination region of a patient during a continuous travel of the examination region through a magnetic resonance apparatus for the generation of an image data set.
- Step 101 the continuous travel of the support device and the acquisition of measurement data 105 during the continuous travel of the support device (and therefore during the continuous travel of the examination subject) is started.
- the examination region that is examined at the beginning of the measurement is an examination region affected by the breathing of the patient
- the patient is already given a breath-hold command (“C”) before the start of the continuous travel and the acquisition of measurement data in Step 101 , i.e. the patient is asked to hold his or her breath, for example for a specific time duration. Otherwise the patient can breathe freely during the acquisition of the measurement data.
- a start according to Step 101 can in particular be initiated precisely like a resumption of the continuous travel that is described later.
- the time duration for which the patient should hold his breath can thereby be largely freely predetermined, and thus be adapted to an individual breath-hold capability of the patient.
- a breath-hold command can thereby require the patient to hold his or her breath “as long as possible”, for example, or until a corresponding, different command to cancel the breath-hold command is given to the patient.
- a maximum breath-hold duration that a patient is capable of therefore determines an upper limit for the predeterminable time duration, which is why the predeterminable time duration depends on the breath-hold duration possible for the patient.
- Such commands to the patient can be given to the patient either by an operator of the magnetic resonance apparatus who is attending the examination or also by an automatic speech output of the magnetic resonance apparatus (insofar as one is present).
- the continuous travel of the support device and the acquisition of measurement data 105 is continued until the continuous travel is either manually interrupted (“m”) or automatically interrupted after a predeterminable time duration (“t”) in a next Step 102 after starting the continuous travel.
- the support device 9 is halted and moved back by a predeterminable distance counter to the travel direction of the continuous travel, meaning that the support device is automatically returned to a new starting position after its continuous travel was stopped.
- Such an interruption 102 of the continuous travel of the support device can be advantageously used for any preparation of an advantageous acquisition of measurement data in the portion of the examination region of the patient that is to be examined after the interruption of the continuous travel.
- a patient can be prepared to hold his or her breath for an acquisition of additional measurement data 106 following the interruption. If the patient should already hold his or her breath before the interruption 102 , the patient can use the interruption 102 in order to breathe freely (“B”), for example until a new breath-hold command (“C”) is given to him.
- the resumption of the continuous travel is initiated (in particular manually (“m”)) and additional measurement data 106 are acquired (Step 103 ).
- An operator of the magnetic resonance apparatus can hereby advantageously wait with the initiation of the resumption of the continuous travel until a patient could prepare for the new acquisition of measurement data, for example until the patient could breathe deeply before a breath-hold interval.
- An extensive gasping for air by the patient for example at the end of a breath-hold interval, which otherwise often leads to artifacts as mentioned above in conventional system, can be avoided in this way. This is in particular avoided in that multiple short breath-hold commands can be given instead of one long one.
- a breath-hold command (“C”) is already given to the patient before the resumption of the continuous travel and the acquisition of measurement data in Step 103 , meaning that the patient is asked to hold his or her breath for a specific time duration, for example.
- the time duration for which the patient should hold his or her breath can be largely freely predetermined as described above, allowing the operator to take into account the individual breath-hold capability of the patient.
- the manual triggering of the resumption of the continuous travel facilitates a coordination of the administering of possible commands to the patient with the movement of the support device 9 and the acquisition of measurement data.
- Step 104 After resumption of the continuous travel it can be checked (Step 104 ) whether an additional interruption for the examination of the examination region is desired, in particular when (for example) measurement data of additional parts of the examination region that are likewise affected by a breathing movement of the patient should be acquired, and whether a new breath-hold command (“C”) must be given in order to prevent movement artifacts in the acquired measurement data. If an additional interruption 102 is desired (“y”), the resumed continuous travel and the acquisition of measurement data 106 can again be interrupted manually (“m”) or automatically after a predeterminable time duration (“t”) after starting the continuous travel (new Step 102 ).
- An image data set (“BDS”) is created from the acquired measurement data 105 and 106 , for example by means of a computer 37 from FIG. 1 .
- Measurement data of an examination region affected by the breathing movement of the patient during a continuous travel of the examination region on a support device can if necessary be acquired in this way with multiple interruptions with a speed of the support device 9 that is optimal for the acquisition or the resolution in MR images reconstructed from the acquired measurement data, even if the acquisition of the measurement data of the examination region takes longer than the patient can hold his or her breath.
- the travel of the support device must initially be braked.
- the support device 9 covers an additional braking path s b in the direction of the continuous travel.
- the support device must likewise be initially accelerated to the speed desired for the continuous travel after initiating a resumption of the continuous travel.
- the support device covers an acceleration path s ac in the direction of the continuous travel.
- An actual return movement path of this distance s in the course of the measurement is advantageously automatically calculated, for example by a computer controlling the magnetic resonance apparatus from the values for s b and s ac .
- the acquisition of the measurement data ensues slice-by-slice, meaning that the examination region is divided into slices in which measurement data are acquired.
- measurement data of the n slices can thereby be acquired, from which n slices measurement data have already been acquired before the interruption of the continuous travel.
- Such “doubly” acquired measurement data can advantageously be used with added weighting in the generation of the image data set, for example in order to correct or to reduce artifacts caused by the interruption by changing the measurement conditions in the magnetic resonance apparatus. Additional exemplary embodiments for this purpose are described further below.
- the distance s is provided as s b +s ac +n*d.
- An actual return travel path of this distance s is advantageously calculated automatically in the course of the measurement, for example by a computer controlling the magnetic resonance apparatus from the values for s b , s ac , n and d.
- the continuous travel can be interrupted arbitrarily often.
- a breath-hold duration given a breath-hold command to the patient can likewise be very freely selected, and therefore the entire measurement can be individually adapted to the capabilities of the patient.
- the time at which an interruption 102 of the continuous travel should be initiated can be determined, for example using an overview image (known as a prescan) of the examination region, analogous to that obtained in a typical planning of an MR examination, for instance under consideration of the dimensions of the examination region and the speed of the continuous travel.
- a prescan an overview image of the examination region
- an operator attending the examination manually initiates an interruption 102 of the continuous travel using a display of the progress of the acquisition of the measurement data during the continuous travel.
- a point in time of an interruption during the continuous travel can thus be controlled interactively by the operator of the magnetic resonance apparatus.
- the operator can recognize from which portion of the examination region measurement data are actually acquired and can decide with this information whether an interruption of the continuous travel—and therefore the acquisition of measurement data for the image data set for a sub-region of the examination region that is to be measured next—is desired, for instance because the patient should hold his breath for the sub-region to be measured next.
- a prevalent patient monitor can be selected, or a current position of the active FOV (i.e. the current position of the acquisition of the measurement data is displayed of the active FOV, thus the current position of the acquisition of the measurement data in a suitable overview image that, for example, was acquired in a conventional manner before the measurement (for instance within the scope of a prescan).
- a display of a current MR image from current or previously acquired measurement data is possible as a display of the progress of the acquisition of the measurement data.
- a current overview image can be calculated (using “maximum intensity projection”, MIP) from current MR images already reconstructed from the measurement data and is displayed to the operator as a projection image.
- FIG. 3 shows a flowchart of an exemplary embodiment of a method to display a progress of an acquisition of measurement data of an examination region of a patient during a continuous travel of the examination region through a magnetic resonance apparatus.
- at least measurement data of the current portion of the examination region that is located in the measurement volume are retrieved from central k-space (Block 202 ) (where the measurement data for the current portion have been entered and stored).
- a current projection image of the current portion of the examination region that is located in the measurement value is calculated (Block 203 ), which currently calculated projection image is displayed on a display device (Block 205 ).
- a current projection image can hereby be calculated via a one-dimensional Fourier transformation along a central k-space line of the currently acquired measurement data, wherein the readout direction is advantageously taken into account in the acquisition of the measurement data.
- That a projection of the measured subject can be acquired by Fourier transformation of a central k-space line (thus a k-space line through the center of k-space), wherein the projection direction in image space is oriented perpendicularly to the k-space line that is used, is already known from the “central slice theorem” (also called “Fourier slice theorem”).
- the central k-space line of a slice of the examination region is also measured in each time interval TS, and normally from the slice that traverses the center of the magnetic resonance apparatus during the time interval TS.
- a projection image of the slice of the examination region that is presently located at a specific location in the magnetic resonance apparatus (for example in its center) can now be calculated per time interval TS from this portion of the acquired measurement data (the measurement data that have been acquired along the central k-space line). It is noted that this projection image can be calculated before the data of the associated slice have been entirely acquired, which is only the case in the SMS technique if the associated slice leaves the active FOV of the magnetic resonance apparatus.
- the calculation of the projection image requires only a one-dimensional Fourier transformation along the central k-space line (one Fourier transformation per coil element used in the event that multiple coil elements are used for the acquisition of the measurement data) and is therefore extremely fast, even independent of whether parallel acquisition and reconstruction techniques that normally increase the reconstruction time are used or not.
- the display of the progress of the acquisition of the measurement data thus changes continuously during the continuous travel, with newly acquired measurement data, and shows in real time from which portion of the examination region current measurement data are acquired.
- a progress image is advantageously constructed (Block 204 ) and displayed (Block 205 ) line-by-line from projection images calculated in succession in the course of the continuous travel of the examination region.
- the progress image can depict a coronal or sagittal projection of the examined examination region, for example.
- each line of the progress image hereby corresponds to a calculated projection image.
- a progress image is thus shown little by little that displays not only the currently measured portion of the examination region but also already measured portions of the examination region. An observer of the progress image therefore receives a more comprehensive impression of the measurement that has already occurred.
- the acquisition of the measurement data ensues slice-by-slice, meaning that the examination region is divided into slices from which measurement data are acquired in succession.
- a projection image can hereby be calculated for every measured slice.
- a line in the progress image then corresponds to a slice of the examination region.
- the progress image can thus already be displayed from the start of the measurement in full size, wherein lines that correspond to slices from which no measurement data have been acquired can, for example, be pre-populated with a pixel value of zero (which corresponds to a grey value of “black”) until a projection image for the corresponding line has been calculated and the progress image is further developed in that the corresponding pixel values of the projection image are adopted for the pixels of the lines of the progress image.
- the progress image is thus continuously updated progressively from the current projection images in the course of the measurement. For example, in the case of the use of the SMS technique the progress image is thus extended once per time interval TS (for example by one additional line) and can be displayed in real time.
- a two-dimensional image for example a progress image
- there are two pixel intervals or spacings thus two spacings between two adjacent pixels, namely one in the column direction and one in the line direction.
- a pixel is normally square, and therefore the pixel spacing in the line direction is the same as that in the column direction.
- the examination region is divided into slices that are normally larger than 1.5625 mm (for example 5 mm), such that a progress image with square pixels in which each line corresponds to a measured slice of the examination region appears to be compressed.
- a pixel spacing between two adjacent lines of the progress image is therefore selected so that it corresponds to the pixel spacing of two adjacent pixels in the readout direction of the examination subject.
- the proportions of the examination region are thus also maintained in the displayed progress image.
- this can be achieved by a linear interpolation.
- a reformation can be implemented so that (for example) approximately 5 mm/1.5625 mm ⁇ 3 lines from the progress image correspond to each slice of the examination region. After such a reformation, the number of lines of the progress image is thus normally different than the number of slices in the examination region.
- the display of a progress image constructed with the use of the “central slice theorem” using the SMS technique can not only ensue in real time but can even proceed “running ahead” since the measurement data for the calculation of a projection image are already available before the entire measurement data of a slice have been acquired.
- the simple (and therefore quickly possible) calculation of the projection images and the simple design of the progress image enables it to not lose this “advance” up to the display of the progress image, even given a use of more complex acquisition techniques and reconstruction methods.
- FIG. 10 shows examples of possible progress images 300 . 1 , 300 . 2 , 300 . 3 at different times during the continuous travel.
- the measurement data have been acquired by means of the aforementioned SMS technique, wherein respectively only the central k-space data that were acquired in the central segments of the FOV were used for the calculation of the projection images.
- the examination region that is to be examined here respectively reaches from the head of the patient (top) to the thighs (bottom).
- a progress image 300 . 1 at an early point in time of the measurement is depicted to the left. As is visible, the progress image 300 . 1 is constructed from the head to the level of the shoulders. The acquisition of the measurement data of the examination region has thus progressed from the head to the level of the shoulders of the patient.
- the remaining lines of the progress image 300 . 1 are still populated with the value of zero and are therefore displayed in black.
- a progress image 300 . 2 at a later point in time during the measurement is depicted in the center.
- the acquisition of the measurement data has already progressed to the level of the hips of the patient.
- measurement data of the entire examination region are acquired in the progress image 300 . 3 depicted to the right, which in turn shows a later point in time just before the end of the measurement, and the progress image was constructed using the projection images of all slices of the examination region.
- the display of a progress image 300 . 1 , 300 . 2 , 300 . 3 advantageously contains additional switching elements SE 1 , SE 2 , SE 3 with which the display and/or the acquisition of the measurement data can be affected manually by activation, for example by clicking with the pointer input device.
- the display of the progress image can be ended by means of the switching element SE 1 , and instead of this the display of another MR image (for example a preceding, reconstructed anatomical MR image data set from the acquired measurement data) can be switched to.
- the acquisition of the measurement data and the continuous travel of the support device can be interrupted and/or resumed by means of switching elements SE 2 and SE 3 .
- an actuation of switching element SE 2 or SE 3 given a continuous travel initiates an interruption of the continuous travel and the acquisition of measurement data.
- the resumption of the continuous travel can then be initiated again (for example via re-actuation of switching element SE 2 ) until either measurement data of the entire examination region have been acquired and the measurement is ended or until the continuous travel and the acquisition of measurement data is interrupted again via re-actuation of switching element SE 2 or SE 3 . If only regions of the examination region that are not affected by the breathing of the patient are still to be measured, the patient can also be instructed to continue to breathe normally before the resumption of the measurement via the switching element SE 2 .
- the operator can also instruct the patient to hold his or her breath in the next resumption of the measurement only as long as it is possible for the patient to do so comfortably, and after this the patient should continue to breathe smoothly and uniformly.
- This procedure prevents the patient from gasping for air at the end of a long breath-hold interval and thus from generating severe breathing movement.
- This procedure additionally has the consequence that this last breath-hold interval normally turns out to be shorter than the preceding, in which a severe “catching of one's breath” has no negative effect on the image quality due to the measurement pauses.
- the switching element SE 2 is reasonably used to start the first measurement phase of the measurement (and therefore to initially start the continuous travel) insofar as the start of the examination region can be freely measured during breathing. For example, this is the case if the measurement begins at the head.
- the resumption of the continuous travel can, for example, be initiated for a predeterminable time duration that, for example, depends on a breath-hold duration that is possible for a patient. This is particularly advantageous if the maximum breath-hold duration of the patient is not sufficient to acquire measurement data of a contiguous portion of the examination region that is affected by the breathing of the patient.
- the measurement and the continuous travel are automatically re-interrupted after the predetermined time duration, the patient can breathe and the measurement can be resumed again after a new breath-hold command in order to measure the entire, contiguous examination region affected by the breathing step-by-step within the scope of the capabilities of the patient.
- the acquisition of measurement data of such a contiguous portion of the examination region that is affected by the breathing of the patient can thus be interrupted as often as necessary and be measured in sub-regions that correspond to the breath-hold capability of the patient.
- An interruption and a resumption of the measurement during continuous travel of the support device or of the examination region can hereby be controlled interactively or semi-automatically (SE 3 ) by an operator.
- Triggering of the resumption of the continuous travel and the measurement normally ensues only after the patient has been given time to breathe and the operator has, if necessary, given a breath-hold command for the acquisition of the measurement data after resumption of the measurement.
- FIGS. 5 through 7 show illustrative diagrams for different exemplary embodiments of the method from FIG. 2 for the acquisition of measurement data of an examination region of a patient during a continuous travel of the examination region through a magnetic resonance apparatus for the generation of an image data set.
- FIGS. 5 through 7 are respectively analogous to the already described FIG. 4 and show exemplary embodiments using the SMS technique for the acquisition of the measurement data.
- FIG. 5 illustrates a particularly temporally efficient embodiment of the method in which the predeterminable distance s by which the support device is moved back after interruption of the continuous travel and before resumption of the continuous travel corresponds precisely to the added distances of a braking path accruing upon interruption of the continuous travel and an acceleration path accruing for the resumption of the continuous travel.
- the measurement is thereby continued with the acquisition of measurement data of those slices and k-space positions corresponding to the sections S 1 , S 2 , S 3 , S 4 that would also have been measured next given an uninterrupted travel.
- the type and severity of the resulting artifacts depends on a number of parameters, for example on the sequence technique that is used and the k-space trajectory that is used. Since, roughly speaking, the measurement data acquired in the acquisition of the measurement data in the center-proximal region of k-space determine the later image contrast and the measurement data acquired in the peripheral region of k-space determine the resolution, given use of the SMS technique the artifacts for such slices in which adjacent k-space lines that lie near the center are acquired in different measurement phases (i.e. before and after an interruption of the measurement and the continuous travel) are particularly noticeable.
- the inner k-space lines are preferably measured in connection with the SMS technique if the appertaining slice occupies the inner sections (thus sections S 2 and S 3 in the shown example) of the active FOV of the magnetic resonance apparatus. If adjacent lines in k-space are measured in temporal succession (known as linear k-space reordering, as used in FLASH sequences, for example) this, in connection with the SMS technique, has the result that a linear correlation between the k-space position and the measurement position in the magnetic resonance apparatus exists. This can be selected so that the inner k-space lines are associated with positions in the center. In FIGS. 4 through 7 , the peripheral k-space lines would thus be associated with the SMS sections S 1 and S 4 and the inner k-space lines would be associated with the SMS sections S 2 and S 3 .
- U 6TS
- the inner k-space lines (in sections S 2 and S 3 ) of the slices 1 and 2 of the second slice stack St e are measured in different measurement phases—i.e. before the interruption U (S 2 ) and after the interruption U (S 3 )—and are therefore particularly prone to artifacts.
- FIG. 6 illustrates an additional embodiment that is less time-efficient but with which the artifacts just mentioned can normally be markedly reduced.
- measurement data are repeatedly measured from the slices that were located in the active FOV of the magnetic resonance apparatus in the last n time intervals of the duration TS before interruption of the measurement.
- total r k-space lines of these slices are respectively measured in both measurement phases, thus before and after the interruption U.
- n ⁇ r 2 ⁇ r k-space lines (that are associated with n SMS sections) of the slices that have been located in the active FOV of the MR system in the two last time intervals of duration TS before interruption of the measurement—thus the slices 5 , 6 , 7 , 8 of the first slice stack St 1 and the slices 1 , 2 of the second slice stack St 2 —are respectively measured in both measurement phases.
- the predeterminable distance s by which the support device is moved back after interruption of the continuous travel and before resumption of the continuous travel can be increased.
- the complete measurement data are acquired for every slice of the complete measurement data at least in one phase with continuous travel, whereby the described artifacts are avoided again.
- a smaller distance s in particular supports the temporal efficiency of the method and a larger distance s reduces artifacts more and more.
- An actual return movement path of the distance s that is predetermined from the cited interval is advantageously calculated automatically from the values for s b , s ac , n and d during a measurement, for example by a computer controlling the magnetic resonance apparatus.
- Measurement data measured repeatedly can already be used to reduce the mentioned artifacts. Multiple possibilities hereby exist.
- s i,p (k y ,k x ) designates the composite measurement data set that is subsequently additionally processed;
- s i,p a (k y ,k x ) stands for the measurement data of the slice with slice index p and stack index i (1 ⁇ p ⁇ 8, 1 ⁇ i ⁇ 3 in FIGS. 5 through 7 ) that are acquired in the first measurement phase (before the interruption U) and
- s i,p b (k y ,k x ) stands for the measurement data of the slice with slice index p and stack index i (1 ⁇ p ⁇ 8, 1 ⁇ i ⁇ 3 in FIGS. 5 through 7 ) that are acquired in the second measurement phase (after the interruption U).
- w i,p (k y ) is a function that assumes the values between zero and one, and k y determines the relative weighting between second and first measurement phase for each k-space line.
- FIGS. 8 and 9 show possible weighting functions w i,p (k y ) for a weighted added use of repeatedly acquired measurement data for the generation of the image data set.
- FIG. 8 shows an example for selection of the weighting function w 1,7 and w 1,8 of slices 7 and 8 of the first slice stack St 1 from FIG. 7 .
- Linear k-space reordering is thereby assumed.
- the k-space lines that are associated with the first section S 1 of the active FOV are measured only in the first measurement phase for these two slices.
- the value of the weighting functions w 1,7 and w 1,8 for these k-space lines is accordingly equal to zero.
- the central k-space lines that are associated with sections S 2 and S 3 are measured during both measurement phases.
- the weighting functions w 1,7 and w 1,8 increases from zero to one in the shape of a cosine function in this range.
- the k-space lines that are associated with the fourth section S 4 are measured only in the second measurement phase.
- the weighting function w 1,7 or, respectively, w 1,8 is accordingly equal to one in this range.
- FIG. 9 shows as an example a possible, corresponding selection of the weighting functions w 2,1 and w 2,2 of slices 1 , 2 of the second slice stack St 2 from FIG. 7 .
- the k-space lines associated with sections S 1 and S 2 of the active FOV are measured repeatedly. Therefore the weighting functions w 2,1 and w 2,2 in this range increase in the shape of a cosine function from zero to one.
- the k-space lines that are associated with the third and fourth section S 3 and S 4 are measured only in the second measurement phase.
- the weighting functions w 2,1 and w 2,2 in this range are accordingly equal to one.
- Measurement data measured repeatedly i.e. before and after the interruption U
- Measurement data measured repeatedly can be used for additional purposes.
- repeatedly measured measurement data can be used in order to establish with image processing techniques whether the patient has realized similar breath-hold states (thus similar positions of his diaphragm) or not in both measurement phases. If the diaphragm position in both measurement phases is similar, the measurement data are identical apart from effects as a result of the transcendent state of the magnetization at the beginning of the second measurement phase and physiological procedures such as heart movement or peristalsis. In this case a degree of correlation either directly between the doubly measured central k-space data or between images that are calculated from these data with the aid of a Fourier transformation is large enough. Conversely, different breath-hold positions can be concluded from a low value of the degree of correlation.
- this information can then be used to automatically select the optimal weighting function, for example for the slices 1 , 2 of the second slice stack St 2 from FIG. 7 .
- repeatedly measured measurement data can be used to respectively reconstruct an MR image with different weightings, for example from the respective acquired central k-space data.
- a first MR image can be reconstructed whereby measurement data acquired in the first measurement phase before the interruption is given a high weighting and a second MR image can be reconstructed with the measurement data acquired in the second measurement phase, after the interruption, being given a high weighting.
- gaps in the examination region that arise due to these different breath-hold positions can be closed or reduced. The danger that, for example, a lesion is overlooked in such a gap can therefore be reduced.
- a degree of correlation between repeatedly measured measurement data of a slice can be calculated and—insofar as the corresponding k-space data were also acquired in a neighboring slice adjacent to this slice—a degree of correlation between these nominally different slices—thus for example the slice and its neighboring slice—can furthermore be calculated. If the calculated degree of correlation between two nominally different slices is greater than the degree of correlation between nominally identically slices, this indicates that the slice of the examination region has migrated due to the different breath-hold position in the magnetic resonance apparatus (independent of the continuous travel). In such a case, the composition of measurement data that were measured at nominally different positions before and after the interruption possibly leads to a better result than the composition of measurement data that were measured at nominally identical slice positions.
- ⁇ , ⁇ tilde over (p) ⁇ is thereby the stack index or slice index of the slice whose measurement data measured before the interruption maximizes the degree of correlation with corresponding measurement data of the slice with stack index i and slice index p that are measured after the interruption.
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Abstract
Description
N=p·S, p≧1 (1)
s i,p(k y ,k x)=(1−w i,p(k y))·s i,p a(k y ,k x)+w i,p(k y)·s i,p b(k y ,k x), 0≦wi,p≦1 (3)
-
- a. wi,p(ky)=0 for those k-space lines ky that are measured only in the first measurement phase.
- b. wi,p(ky)=1 for those k-space lines ky that are measured only in the second measurement phase.
- c. The curve of wi,p(ky) is smooth, or, expressed mathematically:
|w i,p(k y+Δky)−w i,p(k y)|<<1, ∀ky,- wherein Δky is the k-space interval of two adjacent lines, the symbol “<<” stands for “small relative to” and the symbol “∀” means “for all”.
- d. wi,p(ky) assumes small values for k-space lines that are measured during the transcendent state at the beginning of the second measurement phase.
w 2,1(k y)=w 2,2(k y)=1, ∀ky.
s i,p(k y ,k x)=(1−w i,p(k y))·s ĩ, {tilde over (p)} a(k y ,k x)+w i,p(k y)·s i,p(k y ,k x), 0≦wi,p≦1.
ĩ, {tilde over (p)} is thereby the stack index or slice index of the slice whose measurement data measured before the interruption maximizes the degree of correlation with corresponding measurement data of the slice with stack index i and slice index p that are measured after the interruption.
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DE102011005726B4 (en) * | 2011-03-17 | 2012-12-27 | Siemens Aktiengesellschaft | Adjustment of at least one shim current and an associated RF center frequency in a magnetic resonance apparatus during an interleaved multilayer MR measurement of a moving examination subject |
DE102011081411B4 (en) * | 2011-08-23 | 2013-04-11 | Friedrich-Alexander-Universität Erlangen-Nürnberg | Scanning patterns for iterative MR reconstruction methods |
US9569863B2 (en) * | 2012-08-06 | 2017-02-14 | Siemens Healthcare Gmbh | System for accelerated segmented MR image data acquisition |
DE102013205830B4 (en) | 2013-04-03 | 2024-05-29 | Siemens Healthineers Ag | Method and image data generation device for generating image data of a moving object, magnetic resonance system and computer program product |
DE102013205868B4 (en) | 2013-04-03 | 2014-11-27 | Albert-Ludwigs-Universität Freiburg | Method for assigning k-space lines to echo trains and method for acquiring MR data and correspondingly configured magnetic resonance systems |
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US9364166B2 (en) * | 2008-10-03 | 2016-06-14 | Hitachi, Ltd. | Magnetic resonance imaging apparatus and breath-holding imaging method |
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US20100264924A1 (en) | 2010-10-21 |
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CN101862192A (en) | 2010-10-20 |
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